1. Field of the Invention
The invention is directed to an acoustic lens of the type for the transformation of a planar acoustic wave into a spherical segment-shaped acoustic wave and vice versa, for use with media in which the planar or spherical segment-shaped waves propagate having a sound propagation speed therein which is lower than the sound propagation speed in the lens material.
2. Description of the Prior Art
A number of proposals for realizing a lens of the type generally described above are known from the publication "Das elktromagnetische Stosswellensystem von Siemens", by G. Buchholtz et al., Siemens AG, Bereich Medizinische Technik, November 1991, pages 29-33. For example, lenses are known wherein the lens surface adjoining the medium wherein the planar wave propagates is spherically fashioned. In this case, the lens surface that adjoins the medium wherein the spherical segment-shaped wave propagates can be spherically or elliptically fashioned. An elliptical fashioning of both lens surfaces is also known. Further, it is known to fashion the lens surface adjoining the medium wherein the planar wave propagates hyperbolically and to fashion the other lens surface spherically. More or less pronounced aberrations occur in the case of all these lens geometries.
German OS DE 39 24 919 also discloses an acoustic lens having one lens surface formed by two different aspherical surfaces.
Further, U.S. Pat. No. 4,844,198 discloses a focusing arrangement for planar acoustic waves which has two reflector surfaces, one having a parabolic shape and the other having an elliptical shape.
Further, U.S. Pat. No. 4,553,629 discloses an acoustic lens having an elliptical contour with lens surfaces defined by a system of non-linear, partial differential equations.